Graphene is a two-dimensional sheet of graphite, consisting of one to ten layers of carbon atoms arranged in hexagonal lattices. Only six years after first fabricating graphene in the laboratory, Geim and Novoselov were awarded the Nobel Prize in physics for their work on graphene.[1] This relatively short time between discovery and recognition is due in part to the fact that graphene was extensively studied theoretically long before it was discovered experimentally. The greatest obstacle to experimental discovery of graphene was the difficulty detecting the graphene sheets. A single-layer graphene is only ˜0.4 nm thick[2] and absorbs only 2.3% of incident light.[3] This difficulty was overcome in 2004, when the first graphene sheets were created by mechanical exfoliation of highly ordered graphite and visualized by immobilizing the sheets on oxidized silicon (Si/SiO2) wafers. Light interference in the oxide layer (typically 300 nm thick) changes when it is covered by graphene, allowing identification of the graphene on the substrate through color contrast in the reflected light.[2]
The exceptional electrical, optical, and mechanical properties of graphene make it a promising material for many industrial applications such as solar cells, semiconductor devices, and thermal heat sinks.[4,5] However, the greatest obstacle in its use in industry is high-throughput scaling of the production and characterization of graphene. High-throughput production of graphene can be achieved by growing graphene via chemical vapor deposition (CVD) of carbon atoms on metallic substrates.[6-9] Graphene creation using mechanical exfoliation is labor intensive and only produces a few small graphene samples whereas the size of CVD-grown graphene is only limited by the size of the growth chamber.[10] CVD grown graphene has been developed for many different industrial applications, such as electronic devices, [11-13] solar cells, [14,15] and energy storage.[16] The layer thickness and uniformity of a graphene sample are important parameters that affect the performance and properties of the sample. Additionally, cracks and wrinkles in the graphene sample cause variations in the electronic properties that are unrelated to the quality or thickness of the graphene. These defects are difficult to completely avoid due to complicated growth and processing procedures. Therefore, a high-throughput metrology technique that characterizes an entire CVD grown graphene sample is necessary for industrial applications.
The same obstacle that delayed the discovery of graphene makes high-throughput metrology difficult. Common methods for characterizing graphene thickness are Raman microscopy[17] and atomic force microscopy.[18] While these techniques offer insight into the atomic-scale quality of graphene samples, they are slow and limited to characterizing small regions. To overcome these issues, large-scale optical graphene metrology techniques have been developed that identify the layers of graphene immobilized Si/SiO2 substrates based on their color contrast.[6,19] Although Si/SiO2 substrates offer a simple method for improving the visibility of graphene, they complicate the development of a metrology technique suitably robust for industrial use. This is due to the fact that the color sensitivity of cameras changes between camera models and depends on the illumination intensity. Therefore, the color contrast used to identify graphene layers changes between microscopes and slowly changes on the same microscope as the illumination intensity varies. Additionally, the maximum ideal contrast between graphene layers is limited to ˜12%.[20] Therefore, metrology techniques that rely on Si/SiO2 must be calibrated often, a step that requires Raman spectroscopy to identify each individual graphene layer. Finally, many industrial applications, including solar cells and electronic systems on PCB boards, require metrology measurements of graphene samples on substrates other than Si/SiO2.